An isolated plasma membrane vesicle transport system has been developed in order to evaluate the role of regulation of nutrient uptake in growth control in normal and oncogenically transformed cells. This system has led to identification that: 1. Altered rates of uridine transport in confluent fibroblast cultures are a consequence of changes in the membrane itself. 2. Biochemical elucidation of a group translocation transport mechanism in the uptake of the ribose moiety of inosine mediated by membrane nucleoside phosphorylase which is being confirmed genetically through selection and study of a phosphorylase deficient mutant. 3. Amino acid transport across the membrane is responsive to both Na ion stimulation and membrane alterations with growth and oncogenic transformation. In addition, major improvements in membrane isolation procedures have been achieved. The future work will continue to emphasize identifying environmental factors in membrane alterations in transport, selection and characterization of a variety of membrane-enzyme-transport mutants and analysis of diverse nutrient transport mechanisms operating on the membrane at the molecular level.